17
7) The procedure can be repeated with a second standard
addition to verify slope and operation of the method.
Note:
This method is preprogrammed in the Hanna HI
4222pH/ISE/mV meter, which simplifies the method
greatly and permits repeated determinations easily.
Example:
Ammonia determination with known addition:
1. A 50 mL sample of unknown (V
SAMPLE
) is placed in an
clean vessel with an electrode. 1 mL of ISA is added
to the sample and the color change is noted. The
sample is covered and permitted to mix. The mV is
then recorded when the sensor has stabilized.
2. 5 mL (V
STANDARD
) of 0.1 M (C
STANDARD
) standard is then
added to the vessel and is permitted to mix. The mV
value decreases as the concentration increases. (Note:
for other concentration samples, add a known volume
and concentration of standard to produce a 30 mV
change or greater.
3. The unknown ammonia concentration in the original
sample (C
SAMPLE
) can then be determined by using the
equation provided.
